This invention relates to a pressure regulating valve for use in power shift transmissions.
The power shift transmission comprises plural rows of planet gears and is adapted to generate a power changed into a predetermined speed stage by selectively fixing each row of ring gears by means of a hydraulically actuated clutch.
In this type of power shift transmission, the arrangement is made such that the hydraulic pressure for actuating the clutch can be modulated as shown in FIG. 1 in order to relax the peak torque and the speed change shock which occur during the speed change stage. In FIG. 1, reference character T.sub.f is the filling time required to fill the clutch cylinder with oil; that is, a time lag during which no power transmission occurs and T.sub.b is the build-up time during which the clutch is gradually engaged from the semi-clutch condition. Reference character P.sub.f denotes the initial incoming fluid pressure into the clutch cylinder, P.sub.c the initial clutch pressure at the time of commencement of the pressure build-up, and P.sub.s the clutch operating pressure at the time of completion of the pressure build-up. In the build-up time T.sub.b, the clutch is smoothly rendered operative so as to enable the relaxation of the peak torque and the reduction of the shock generating at the time of speed changes to be achieved.
As mentioned above, the conventional power shift transmission comprises a plurality of planet gear trains each having a clutch; however, each clutch has a different number and size of clutch disks, respectively, from the viewpoint of the durability thereof.
For this reason, the stroke and area of the pistons each slidably mounted within the respective hydraulic cylinders for urging the disks of the clutch are different. Therefore, each of the clutches has different cylinder volume and in some cases the largest clutch has a cylinder volume as about three times that of the smallest clutch. Further, each of the clutches has a different conduit frictional resistance between the respective cylinders and the modulation valves for modulating or gradually increasing the pressure of the fluid flowing into the cylinders. Further, all the volume delivered by the pump is not supplied into the clutch cylinder because part of the volume of the fluid is relieved through the modulation valve and so the filling time tends to increase.
For this reason, as shown in FIG. 2, even in a single power shift transmission, the filling time T.sub.f1 and T.sub.f2 differ by speed stages.
The large variations in filling time caused by speed changes will give an unpleasant feeling to the driver. Further, where the filling time is long and particularly when it is desired to accelerate the vehicle, the clutch remains disengaged even with the accelerator pedal being depressed during the filling time so that no load will be imposed on the engine during that time and therefore the engine will suddenly rotate very rapidly. Whilst, because no power is transmitted to the vehicle driven during this period, the speed of the vehicle is reduced due to the running resistance and the gradient resistance. Since the clutch commences engagement from this state, the clutch must absorb or take up an extra work due to the racing of the engine and the reduction in the vehicle speed, thus causing damage to the clutch. Further, such engagement of the clutch will cause sudden changes in the engine speed and also in the acceleration of the vehicle, and therefore the driver will experience great shocks. Further, in case where the speed of the vehicle is reduced so much due to a large filling time, the allowance torque for the acceleration becomes smaller in high speed stages and so the acceleration takes a longer time. If the filling time is long, when the vehicle is climbing a slope, it tends to slip down.
FIG. 3 shows how engine revolution and vehicle speed are changed in accordance with fluid pressure changes at clutches having different filling times. For a clutch having filling time T.sub.f1 which is shorter than T.sub.f2, the racing of the engine becomes small and the decrease of the vehicle speed becomes small, whilst for a clutch having filling time T.sub.f2, the racing of the engine becomes large and the decrease of the vehicle speed becomes large. With large drop in the vehicle speed, it takes a longer time to accelerate the vehicle thereafter.
In such a case, it is required to take measures to reduce the filling time. One conventional solution to this problem is to reduce the amount of relief of the pressurized fluid from the modulation valve during filling operation and supply the fluid under pressure delivered by the pump effectively into the clutch cyliner.
For this purpose, it is only necessary to raise the initial clutch pressure P.sub.c of the modulation valve. However, if a speed change occurs between drive ratios having extremely different filling times as shown in FIG. 2, then a problem will occur which is mentioned below.
In brief, if the initial clutch pressure is increased from P.sub.c to P.sub.c ' in order to reduce the filling time from T.sub.f2 to T.sub.f2 ', then the shorter filling time T.sub.f1 is reduced further, and therefore the fluid pressure begins to increase again while it has not lowered enough to its predetermined value thereby causing a poor modulation so that it becomes unable to obtain a proper modulation effect for relaxing the peak torque and reducing the shocks during speed changes.